2023, September 4: Planet Loop-to-Loop

September 4, 2023: Venus’ retrograde ends while Jupiter begins to move westward against the starry background.

Jupiter’s retrograde motion against the starfield is demonstrated for 2023.
Chart Caption – Jupiter’s retrograde motion against the starfield is demonstrated for 2023.

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by Jeffrey L. Hunt

Chicago, Illinois:  Sunrise, 6:19 a.m. CDT; Sunset, 7:19 p.m. CDT.  Check local sources for sunrise and sunset times. Times are calculated by the US Naval Observatory’s MICA computer program.

Today daylight shrinks to thirteen hours.  During the next three weeks, daylight loses an hour as the equinox passes.

The five-planet morning parade continues with Venus, Jupiter, and Saturn easily visible.  Uranus is easy to locate through a binocular between Jupiter and the Pleiades star cluster.  The challenging view, even through a binocular, is locating Neptune in a dim Pisces starfield, over 20° to the upper left of Saturn.  For sky watchers interested in seeing the two more distant planets, see the directions in the August 27th article.

When Venus passes from the evening sky – east of the sun – to the morning sky – west of the sun – it passes inside Earth’s orbit.  This westward motion is known as retrograde motion.  The sun is the main star to reference the planet’s motion. In the evening sky, Mercury is quickly moving westward or retrograde. The inner planets’retrogrades are from their faster motion as they pass inferior conjunction.

Venus’ retrograde ends 86.5° east of Jupiter and 19.3° west of Regulus, Leo’s brightest star, that is emerging from the sun for its first morning appearance. Venus moves eastward and passes Regulus on October 10th.

Celestial bodies farther from the sun than Earth retrograde against the starry background, but for a different reason. This is from Earth overtaking them on an inside orbital path.

The outer planets revolve around the sun at a slower pace.  Depending on their distance from the sun, Earth laps them at regular intervals.  Saturn, the most distant bright planet that can be seen without an optical assist, revolves around the sun nearly every 30 years.  Earth catches the planet, passing between the sun and it every 378 days. For Jupiter, this occurs about every 400 days. 

Mars is closer to the sun than the Jovian pair and revolves around the central star about every two years in rough figures. Earth passed Mars December 8, 2022, and laps the Red Planet again January 16, 2025, an interval of 761 days.

As Earth overtakes the bright outer planets, they seem to stop moving eastward against the stars – their usual direction.  As Earth goes in between the planet and the sun, the distant world – appearing as a star – seems to back up compared to the fixed patterns of the constellations.

Earth is between Jupiter and the sun, known as opposition, on November 3rd.  The planet’s retrograde ends December 30th. 

On the accompanying chart, location “A” indicates when Jupiter’s retrograde begins.  At “B,” Jupiter is at opposition and appeared to move westward compared to the starry background shown by the line of sight from Earth to Jupiter and projected toward the distant stars.  At “C,” retrograde ends and the planet appears to move eastward again. Notice on the diagram that Jupiter moves eastward along its orbit, but westward against the starry background from Earth’s perspective.

Summaries of Current Sky Events

Summary for Venus as a Morning Star, 2023-24

Here is today’s planet forecast:

Morning Sky

2023, September 4: Jupiter and the gibbous moon are high in the southern sky during morning twilight.
Chart Caption – 2023, September 4: Jupiter and the gibbous moon are high in the southern sky during morning twilight.

Find brilliant Venus and Jupiter during morning twilight. Jupiter is that bright star to the upper left of the moon this morning.  The separation is only 5.1°.  A binocular might be needed to see the star Hamal, 13.6° to the upper right of the planet.

2023, September 4: Morning Star Venus, Sirius, and Procyon are in the eastern sky before sunrise.
Chart Caption – 2023, September 4: Morning Star Venus, Sirius, and Procyon are in the eastern sky before sunrise.

About forty-five minutes before daybreak, the Morning Star is over 15° above the east horizon.  It is nearly the same altitude – height above the horizon – as Sirius that is in the southeastern sky.  The brightest planet and the night’s brightest star are nearly the same altitude in the morning sky.  Venus does not pass closely to Sirius, so there is no conjunction.  Look for them at about the same altitude during morning twilight throughout the month.

Look for the star Procyon, the Little Dog Star, above an imaginary line from Venus to Sirius.  Procyon’s name means “before the dog,” because it rises less than thirty minutes before the Dog Star, Sirius, from mid-northern latitudes.

 Evening Sky

Mercury as Never Seen Before
Photo Caption – Mercury as Never Seen Before. (NASA photo)

Mercury and Mars are immersed in bright sunlight.  As described previously, Mercury is retrograding as it moves from the evening sky to a morning view. It passes between Earth and the sun in two days, heading for its best morning appearance of the year.

Mars
Mars (NASA)

Mars is dimmer than is expected, setting less than an hour after sunset.

2023, September 4: Saturn is in the southeast with Skat and Lambda Aquarii (λ Aqr) during the early evening.
Chart Caption – 2023, September 4: Saturn is in the southeast with Skat and Lambda Aquarii (λ Aqr) during the early evening.

Saturn is above the east-southeast horizon at sunset.  Wait until the sky is darker and the planet is higher in the southeastern sky to see it.  Two hours after sundown, the Ringed Wonder is over 20° above the horizon, 8.8° to the upper right of Skat, the Aquarian’s leg, and 8.8° to the right of Lambda Aquarii (λ Aqr on the chart).  Together the trio nearly makes an equilateral triangle.  Use a binocular to find the starfield.

2023, September 4: As midnight nears, Jupiter and the gibbous moon are in the eastern sky.
Chart Caption – 2023, September 4: As midnight nears, Jupiter and the gibbous moon are in the eastern sky.

Jupiter rises less than three hours after nightfall.  As the midnight hour approaches, the gibbous moon, 67% illuminated, is about 15° above the east-northeast horizon, and 6.9° to the lower left of bright Jupiter.

By morning twilight, Jupiter and Moon are high in the southern sky and Venus appears in the eastern sky.

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